1. Field of Invention
The present invention is directed to continuous casting slab suitable for the production of non-tempered high tensile steel materials having high tensile strength and excellent toughness. The present invention is also directed to a method of manufacturing non-tempered high tensile steel materials using the casting slab as the raw material.
2. Description of Related Art
As a method of manufacturing steel materials having characteristics such as strength, toughness and weldability in good balance, a method of refining structures with TMCP (Thermo-Mechanical Control Process) is known.
However, for fully providing the effect of rolling in a non-recrystallization temperature range to attain the refinement of the structures by such a method, large rolling reduction must be applied at a low temperature range. This results in problems such as (a) a large load is imposed on a rolling mill, (b) a sufficient draft ratio cannot be ensured for materials of large thickness, and (c) waiting time for the temperature control increases to lower the rolling efficiency. Unless such problems are overcome, no effective refinement of the structures can be attained that also improves the characteristics such as strength, toughness and weldability.
In addition to the refinement of the structures, a technique is known that utilizes the function of forming intra-granular ferrite nuclei and the precipitation hardening function of VN (vanadium nitride) precipitated in steels. For example, Japanese Patent Publication No. 2368/1964 and the Report of Japan Iron and Steel Society (Iron and Steel, vol. 77, 1991, No. 1, page 171) disclose the technique of refining the structures by adding a large amount of N together with V to improve the strength and the toughness.
In addition, Japanese Patent Laid-Open No. 186848/1989 discloses a technique of dispersing composite precipitates of TiN-MnS-VN with the addition of Ti, thereby effectively providing the ferrite forming function with VN acting as ferrite nucleation site, thereby improving the toughness in weld heat affected zones. Further, Japanese Patent Laid-Open No. 125140/1997 (U.S. Pat. No. 5,743,972) discloses a method of manufacturing wide beam flanges of large thickness that is excellent in toughness and material homogeneity by the composite addition of V and N and by ferrite grain size control.
However, in the case of continuously casting V (vanadium) containing steel slabs, cracks such as transverse facial cracks or corner cracks tend to occur on the surface of the casting slab upon bending or unbending. These cracks make it difficult to obtain continuous casting slabs of excellent surface quality. If such cracks are formed on the surface of the continuous casting slab, a direct rolling process of directly feeding high-temperature continuous casting slabs with no surface treatment to a hot rolling step cannot be applied and the production cost consequently increases. For preventing surface cracks in continuous casting slabs of V containing steels, it has been known to be effective to reduce the N (nitrogen) content and, further, forming TiN with the addition of Ti, thereby trapping N. However, because the amount of N in the steels required for forming VN is insufficient in such methods, the function of forming intra-granular ferrite nuclei for VN and precipitation hardening ability cannot be utilized effectively.
In view of the above-described problems of the known art, an object of the present invention is to provide a continuous casting slab with no surface cracks while containing VN in the steels.
It is also an object of the present invention to provide a method of manufacturing non-tempered high tensile steel materials having favorable toughness by using the continuous casting slab.
The material properties that can be provided in embodiments of the steel materials according to the invention are: yield strength (YS) of about 325 MPa or more, tensile strength (TS) of about 490 MPa or more, Charpy impact absorption energy at xe2x88x9220xc2x0 C. (vE-20) of about 200 J or more, and impact absorption energy at 0xc2x0 C. (vE0) in weld heat affected zones of about 110 J or more. In some preferred embodiments of the steel materials, the tensile strength can be 520 MPa or more.
The present inventors have attained a compatibility between the material properties and the inhibition of surface cracks of the casting slab that has been difficult to obtain. Particularly, by controlling the steel composition, and by controlling the relation between each of the specific components of the compositions, the precipitation of VN and MnS can be controlled.
The invention provides a steel continuous casting slab with no surface cracks comprising: C: about 0.05 to 0.18 wt %, Si: about 0.6 wt % or less, Mn; about 0.80 to 1.80 wt %, P: about 0.030 wt % or less, S: about 0.004 wt % or less, Al: about 0.050 wt % or less, V: about 0.04 to 0.15 wt % and N: about 0.0050 to 0.0150 wt %, and at least one of Ti: about 0.004 to 0.030 wt % and B: about 0.0003 to 0.0030 wt % within a range satisfying the equation (1) below; and further comprising at least one of Ca: about 0.0010 to 0.0100 wt % and REM: about 0.0010 to 0.0100 wt % within a range satisfying the equation (2) below; with the balance being iron and inevitable impurities:
5.0xe2x89xa6[V](wt %)/([N](wt %)xe2x88x920.292xc3x97[Ti](wt %)xe2x88x921.295xc3x97[B](wt %))xe2x89xa618.0xe2x80x83xe2x80x83(1)
[Mn](wt %)xc3x97([S](wt %)xe2x88x920.8xc3x97([Ca](wt %)xe2x88x92110xc3x97[Ca](wt %)xc3x97[O](wt %))xe2x88x920.25xc3x97([REM](wt %)xe2x88x9270xc3x97[REM](wt %)xc3x97[O](wt %)))xc3x97103xe2x89xa61.0xe2x80x83xe2x80x83(2)
Further, the steel can also comprise Cu, Ni, Cr, Mo and Nb.
Further, the invention provides a method of manufacturing non-tempered high tensile steel materials. Exemplary embodiments of the method according to the invention comprise heating the continuous casting slab at a temperature of from about 1050xc2x0 to 1250xc2x0 C., and applying hot working with a cumulative draft of 30% or more within a temperature range of from about 1050xc2x0 to 950xc2x0 C.